1. Field of the Invention
The present invention relates to a hydrocarbon sensor for detecting hydrocarbon and measuring its concentration in a temperature range from room temperature to high temperature (800.degree. C.) that can be used for combustion control (clean burn) of combustion engine and appliance, for detecting hydrocarbon in exhaust gas from automotive engine, stove, and catalytic combustion appliance, by detection of hydrocarbon in the living environments.
2. Related Art of the Invention
As the method for measuring or detecting hydrocarbons, roughly classified by material, the semiconductor type and electrolyte type are known. In the semiconductor type using TiO.sub.2, SnO.sub.2, and the like, a semiconductor material is blended with a catalytic active material in order to react with CO and other reducing gas and to add selectivity. For combustion with catalyst, however, the atmosphere requires oxygen, and the hydrocarbon concentration cannot be detected correctly in oxygen-free state or in the atmosphere varying in the oxygen concentration.
The type using electrolyte, on the other hand, requires an excellent proton conductor. For use in combustion engine and appliance, a proton conductor of oxide usable at room temperature or higher is needed. Recently, as proton conductor of oxide, CaZr.sub.0.9 In.sub.0.1 O.sub.3-.alpha. oxide is developed, and it is attempted to be applied in hydrocarbon sensor.
As the hydrocarbon sensor using solid electrolyte of calcium-zirconium oxide as far as known at the present, the electromotive force type using Pd-Au electrode (Nagoya University) &lt;reference, Collected papers of spring meeting of Japan Electrochemical Society, 1994&gt;, and the limit current detection type using porous alumina as diffusion rate determining layer (Toyota Central Research Institute) &lt;reference, Collected paper of fall meeting of Japan Sensor Association, 1995&gt; are known. However, the proton conductivity of solid electrolyte of calcium-zirconium oxide is as small as 5.times.10.sup.-4 S/cm at 600.degree. C., and in order to raise the sensor sensitivity, in the electromotive force type, the operating temperature must be set at high temperature of 700.degree. C., or in the current detection type, it is difficult to use unless the film is made thin, and a solid electrolyte material of a higher proton conductivity has been demanded.
As for detecting mechanism and structure, in the electromotive force type, since the catalytic function of electrode is utilized, accurate detection of hydrocarbon is not expected in oxygen-free state or in the atmosphere of large concentration changes of oxygen. In the current type using alumina porous matter in the diffusion rate determining layer, it is hard to set the electrolyte voltage of hydrocarbon. Thus, various problems are involved in the prior arts.
That is, as the detector of hydrocarbons in living environments, or the hydrocarbon concentration detector of combustion exhaust gas from automotive engine, stove and other combustion appliance, there is an increasing demand for small, handy, and inexpensive sensor that is selective, and high in sensitivity and reliability in any atmosphere (free from effects of oxygen concentration). The sensor using calcium-zirconium oxide is small in proton conductivity, and a solid electrolyte having a higher proton conductivity is needed for smaller size, easier use, and higher sensitivity. Considering the detection characteristic, the method and mechanism for detecting hydrocarbons only accurately in any ambient environments are demanded.